1. Field of Invention
This invention relates to an electronic caliper. More particularly, this invention is directed to electronic calipers using a reduced offset induced current position transducer.
2. Description of Related Art
U.S. patent application Ser. No. 08/645,483 filed May 13, 1996, and incorporated herein in its entirety, discloses an electronic caliper using an inductive position transducer.
The operation of the electronic caliper using the inductive position transducer described in the application Ser. No. '483 is generally shown in FIGS. 1, 2, and 3. As shown in FIG. 1, an inductive caliper 100 includes an elongated beam 102. The elongated beam 102 is a rigid or semi-rigid bar having a generally rectangular cross section. A groove 106 is formed in an upper surface of the elongated beam 102. An elongated measuring scale 104 is rigidly bonded to the elongated beam 102 in the groove 106. The groove 106 is formed in the beam 102 at a depth about equal to the thickness of the scale 104. Thus, the top surface of the scale 104 is very nearly coplanar with the top edges of beam 102.
A pair of laterally projecting, fixed jaws 108 and 110 are integrally formed near a first end 112 of the beam 102. A corresponding pair of laterally projecting movable jaws 116 and 118 are formed on a slider assembly 120. The outside dimensions of an object are measured by placing the object between a pair of engagement surfaces 114 on the jaws 108 and 116. Similarly, the inside dimensions of an object are measured by placing the jaws 110 and 118 within an object. The engagement surfaces 122 of the jaws 110 and 118 are positioned to contact the surfaces on the object to be measured.
The engagement surfaces 122 and 114 are positioned so that when the engagement surfaces 114 of the jaws 108 and 116 are contacting each other, the engagement surfaces 122 of the jaws 110 and 118 are aligned with each other. In this position, the zero position (not shown), both the outside and inside dimensions measured by the caliper 100 should be zero.
The caliper 100 also includes a depth bar 124 which is attached to the slider assembly 120. The depth bar 124 projects longitudinally from the beam 102 and terminates at an engagement end 126. The length of the depth bar 124 is such that the engagement end 126 is flush with a second end 128 of the beam 102 when the caliper 100 is at the zero position. By resting the second end 128 of the beam 102 on a surface in which a hole is formed and extending the depth bar 124 into the hole until the end 126 touches the bottom of the hole, the caliper 100 is able to measure the depth of the hole.
Whether a measurement is made using the outside measuring jaws 108 and 116, the inside measuring jaws 110 and 118, or the depth bar 124, the measured dimension is displayed on a conventional digital display 134, which is mounted in a cover 136 of the caliper 100. A pair of push button switches 130 and 132 are also mounted in the cover 136. The switch 130 turns on and off a signal processing and display electronic circuit 160 of the slider assembly 120. The switch 132 is used to reset the display 134 to zero.
As shown in FIG. 1, the slider assembly 120 includes a base 138 with a guiding edge 140. The guiding edge 140 contacts a side edge 146 of the elongated beam 102 when the slider assembly 120 straddles the elongated beam 102. This ensures accurate operation of the caliper 100. A pair of screws 144 bias a resilient pressure bar 146 against a mating edge of the beam 102 to eliminate free play between the slider assembly 120 and the elongated beam 102.
The depth bar 124 is inserted into a depth bar groove 148 formed on an underside of the elongated beam 102. The depth bar groove 148 extends along the underside of the elongated beam 102 to provide clearance for the depth bar 124. The depth bar 124 is held in the depth bar groove 148 by an end stop 150. The end stop 150 is attached to the underside of the beam 102 at the second end 128. The end stop 150 also prevents the slider assembly 120 from inadvertently disengaging from the elongated beam 102 at the second end 128 during operation.
The slider assembly 120 also includes a read head assembly 152 mounted on the base 138 above the elongated beam 102. Thus, the base 138 and read head assembly 152 move as a unit. The read head assembly 152 includes a substrate 154 such as a conventional printed circuit board. The substrate 154 bears an inductive read head 158 on its lower surface. A signal processing and display electronic circuit 160 is mounted on an upper surface of the substrate 154. A resilient seal 156 is compressed between the cover 136 and the substrate 154 to prevent contamination of the signal processing and display electronic circuit 160.
As shown in FIG. 2, the read head 158 is covered by a thin, durable, insulative coating 162, which is preferably approximately 50 microns thick.
The scale 104 is preferably an elongated printed circuit board (PCB) 164. As shown in FIG. 1, a set of magnetic flux modulators 166 are spaced apart along the PCB 164 in a periodic pattern. The flux modulators 166 are preferably formed of copper. The flux modulators 166 are preferably formed according to conventional printed circuit board manufacturing techniques, although many other methods of fabrication may be used. As shown in FIG. 2, a protective insulating layer 168 (preferably being at most 100 microns thick) covers the flux modulators 166. The protective layer 168 can include printed markings, as shown in FIG. 1.
The slider assembly 120 carries the read head 158 so that it is slightly separated from the beam 102 by an air gap 170 formed between the insulative coatings 162 and 168. The air gap 170 is preferably on the order of 0.5 mm. Together, the read head 158 and the flux modulators 166 form an inductive transducer.
As shown in FIG. 3, the magnetic flux modulators 166 are distributed along a measuring axis 174 of the elongated beam 102 at a pitch equal to a wavelength .lambda., which is described in more detail below. The flux modulators 166 have a nominal width along the measuring axis 174 of .lambda./2. The flux modulators 166 have a width d in a direction perpendicular to the measuring axis 174.
The read head 158 includes a generally square transmitter winding 176 that is connected to a drive signal generator 178. The drive signal generator 178 provides a time varying drive signal to the transmitter winding 176. The time varying drive signal preferably results in a sinusoidal signal in the transmitter winding 176, and more preferably an exponentially decaying sinusoidal signal. When the time varying drive signal is applied to the transmitter winding 176, the time varying current flowing in the transmitter winding 176 generates a time varying, or changing, magnetic field. Because the transmitter winding 176 is generally rectangularly shaped, the generated magnetic field is generally constant within a flux region inside the transmitter winding 176.
The read head 158 further includes a first receiver winding 180 and a second receiver winding 182 positioned on the read head 158 within the flux region inside the transmitter winding 176. Each of the first receiver winding 180 and the second receiver winding 182 is formed by a plurality of first loop segments 184 and second loop segments 186. The first loop segments 184 are formed on a first surface of a layer of the printed circuit board 154. The second loop segments 186 are formed on another surface of the layer of the printed circuit board 154. The layer of the printed circuit board 154 acts as an electrical insulation layer between the first loop segments 184 and the second loop segments 186. Each end of the first loop segments 184 is connected to one end of one of the second loop segments 186 through feed-throughs 188 formed in the layer of the printed circuit board 154.
The first and second loop segments 184 and 186 are preferably sinusoidally shaped. Accordingly, as shown in FIG. 3 the first and second loop segments 184 and 186 forming each of the receiver windings 180 and 182 form a sinusoidally shaped periodic pattern having a wavelength .lambda.. Each of the receiver windings 180 and 182 are thus formed having a plurality of loops 190 and 192.
The loops 190 and 192 in each of the first and second receiver windings 180 and 182 have a width along the measuring axis 174 equal to .lambda./2. Thus, each pair of adjacent loops 190 and 192 has a width equal to .lambda.. Furthermore, the first and second loop segments 184 and 186 go through a full sinusoidal cycle in each pair of adjacent loops 190 and 192. Thus, .lambda. corresponds to the sinusoidal wavelength of the first and second receiver windings 180 and 182. Furthermore, the second receiver winding 182 is offset by .lambda./4 from the first receiver winding 180 along the measuring axis 174. That is, the first and second receiver windings 180 and 182 are in quadrature.
The changing drive signal from the drive signal generator 178 is applied to the transmitter winding 176 such that current flows in a transmitter winding 176 from a first terminal 176a, through the transmitter winding 176 and out through a second terminal 176b. Thus, the magnetic field generated by the transmitter winding 176 descends into the plane of FIG. 3 within the transmitter winding 176 and rises up out of the plane of FIG. 3 outside the transmitter winding 176. Accordingly, the changing magnetic field within the transmitter winding 176 generates an induced electromagnetic force (EMF) in each of the loops 190 and 192 formed in the receiver windings 180 and 182.
The loops 190 and 192 have opposite winding directions. Thus, the EMF induced in the loops 190 has a polarity that is opposite to the polarity of the EMF induced in the loops 192. The loops 190 and 192 enclose the same area and thus nominally the same amount of magnetic flux. Therefore, the absolute magnitude of the EMF generated in each of the loops 190 and 192 is nominally the same.
There are preferably equal numbers of loops 190 and 192 in each of the first and second receiver windings 180 and 182. Thus, the positive polarity EMF induced in the loops 190 is exactly offset by the negative polarity EMF induced in the loops 192. Accordingly, the net nominal EMF on each of the first and second receiver windings 180 and 182 is zero. Thus, no signal should be output from the first and second receiver windings 180 and 182 as a result solely of the direct coupling from the transmitter winding 176 to the receiver windings 180 and 182.
When the read head 158 is placed in proximity to the PCB 164, the changing magnetic flux generated by the transmitter winding 176 also passes through the flux modulators 166. The flux modulators 166 modulate the changing magnetic flux and can be either flux enhancers or flux disrupters.
When the flux modulators 166 are provided as flux disrupters, the flux modulators 166 are formed as conductive plates or thin conductive films on the PCB 164. As the changing magnetic flux passes through the conductive plates or thin films, eddy currents are generated in the conductive plates or thin films. These eddy currents in turn generate magnetic fields having a field direction that is opposite to that of the magnetic field generated by the transmitter winding 176. Thus, in areas proximate to each of the flux disrupter-type flux modulators 166, the net magnetic flux is less than the net magnetic flux in areas distant from the flux disrupter type flux modulators 166.
When the scale PCB 164 is positioned relative to the read head 158 such that the flux disrupters 166 are aligned with the positive polarity loops 190 of the receiver winding 180, the net EMF generated in the positive polarity loops 190 is reduced compared to the net EMF generated in the negative polarity loops 192. Thus, the receiver winding 190 becomes unbalanced and has a net negative signal across its output terminals 180a and 180b.
Similarly, when the flux disrupters 166 are aligned with the negative polarity loops 192, the net magnetic flux through the negative polarity loops 192 is disrupted or reduced. Thus, the net EMF generated in the negative polarity loops 192 is reduced relative to the net EMF generated in the positive polarity loops 190. Thus, the first receiver winding 180 has a net positive signal across its output terminals 180a and 180b.
When the flux modulators 166 are provided as flux enhancers, this result is exactly reversed. The flux enhancer type flux modulators 166 are formed by portions of high magnetic permeability material provided in or on the scale member 104, in place of the conductive plates of PCB 164. The magnetic flux generated by the transmitter winding 176 preferentially passes through the high magnetic permeability flux enhancer type flux modulators 166. That is, the density of the magnetic flux within the flux enhancers 166 is enhanced, while the flux density in areas outside the flux enhancers 166 is reduced.
Thus, when the flux enhancers 166 are aligned with the positive polarity loops 190 of the second receiver winding 182, the flux density through the positive polarity loops 190 is greater than a flux density passing through the negative polarity loops 192. Thus, the net EMF generated in the positive polarity 190 increases, while the net EMF induced in the negative polarity loops 192 decreases. This appears as a positive signal across the terminals 182a and 182b of the second receiver winding 182.
When the flux enhancers 166 are aligned with the negative polarity loops 192, the negative polarity loops 192 generate an enhanced EMF relative to the EMF induced in the positive polarity loops 190. Thus, a negative signal appears across the terminals 182a and 182b of the second receiver winding 180. It should also be appreciated that, as outlined in the incorporated reference, both the flux enhancing and flux disrupting effects can be combined in a single scale, where the flux enhancers and flux disrupters are interleaved along the length of the scale 104. This would act to enhance the modulation of the induced EMF, because the effects of both types of flux modulators additively combine.
As indicated above, the width and height of the flux modulators 166 are nominally .lambda./2 and d, respectively, while the pitch of the flux modulators 166 is nominally .lambda.. Similarly, the wavelength of the periodic pattern formed in the first and second receiver windings 180 and 182 is nominally .lambda. and the height of the loops 190 and 192 is nominally d. Furthermore, each of the loops 190 and 192 enclose a nominally constant area.
FIG. 4A shows the position-dependent output from the positive polarity loops 190 as the flux modulators 166 move relative to the positive polarity loops 190. Assuming the flux modulators 166 are flux disrupters, the minimum signal amplitude corresponds to those positions where the flux disrupters 166 exactly align with the positive polarity loops 190, while the maximum amplitude positions correspond to the flux disrupters 166 being aligned with the negative polarity loops 192.
FIG. 4B shows the signal output from each of the negative polarity loops 192. As with the signal shown in FIG. 4A, the minimum signal amplitude corresponds to those positions where the flux disrupters 166 exactly align with the positive polarity loops 190, while the maximum signal output corresponds to those positions where the flux disrupters exactly align with the negative polarity loops 192. It should be appreciated that if flux enhancers were used in place of flux disrupters, the minimum signal amplitudes in FIGS. 4A and 4B would correspond to the flux enhancers 166 aligning with the negative polarity loops 192, while the maximum signal amplitude would correspond to the flux enhancers 166 aligning with the positive polarity loops 190.
FIG. 4C shows the net signal output from either of the first and second receiver windings 180 and 182. This net signal is equal to the sum of the signals output from the positive and negative polarity loops 190 and 192, i.e., the sum of the signal shown in FIGS. 4A and 4B. The net signal shown in FIG. 4C should ideally be symmetrical around zero, that is, the positive and negative polarity loops 190 and 192 should be exactly balanced to produce a symmetrical output with zero offset.
However, a "DC" (position independent) component often appears in the net signal in a real device. This DC component is the offset signal V.sub.o. This offset V.sub.o is an extraneous signal component that complicates signal processing and leads to undesirable position measurement errors. This offset has two sources.
First, the full amplitude of the transmitter field passes through the first and second receiver windings 180 and 182. As outlined above, this induces a voltage in each loop 190 and 192. The induced voltage nominally cancels because the loops 190 and 192 have opposite winding directions. However, to perfectly cancel the induced voltage in the receiver windings requires the positive and negative loops 190 and 192 to be precisely positioned and shaped, for a perfectly balanced result. The tolerances on the balance are critical because the voltages induced directly into the receiver winding loops 180 and 182 by the transmitter winding 176 are much stronger than the modulation in the induced voltage caused by the flux modulators 166.
Second, the spatially modulated field created by the flux modulators also exhibits an average position-independent offset component. That is, the flux modulators 166 within the magnetic field generated by the transmitter winding 176 all create the same polarity spatial modulation in the magnetic field. For example, when flux disrupters are used, the induced eddy current field from the flux modulators has an offset because the flux disrupters within the transmitter field all create a same polarity secondary magnetic field. At the same time, the space between the flux disrupters does not create a secondary magnetic field.
Thus, each positive polarity loop 190 and each negative polarity loop 192 of the receiver windings 180 and 182 sees a net magnetic field that varies between a minimum value and a maximum value having the same polarity. The mean value of this function is not balanced around zero, i.e., it has a large nominal offset. Similarly, when flux enhancers are used, the field modulation due to the flux enhancers has an offset because the enhancers within the transmitter winding 176 all create the same field modulation, while the space between the modulators provides no modulation. Each positive and negative polarity loop 190 and 192 of each receiver winding 180 or 182 therefore sees a modulated field that varies between a minimum value and a maximum value having the same polarity. The mean value of this function also has a large nominal offset.
A receiver winding having an equal number of similar positive and negative polarity loops 190 and 192 helps eliminate the offset components. However, any imperfection in the balance between the positive and negative polarity loops 190 and 192 allows residual offsets according to the previous description.
Both these offset components are expected to be canceled solely by the symmetry between the positive and negative polarity loops 190 and 192 in the first and second windings 180 and 182. This puts very stringent requirements on the manufacturing precision of the receiver windings 180 and 182. Experience in manufacturing a transducer indicates it is practically impossible to eliminate this source of error from the induced current position transducer of a conventional caliper.
Furthermore, any deviations in the width or pitch of the flux modulators 166 will unbalance the receiver windings 180 or 182 in a way that is independent of the relative position between the PCB 164 and the read head 158.
Any signal component which is independent of the transducer position, such as the aforementioned offset components, is regarded as an extraneous signal to the operation of the transducer. Such extraneous signals complicate the required signal processing circuitry and otherwise lead to errors which compromise the accuracy of the transducer.
One proposed solution attempts to reduce the extraneous coupling between the transmitter and receiver windings simply by placing the receiver winding distant from the field produced by the transmitter winding. However, the effectiveness of this technique alone depends on the degree of separation between the transmitter and receiver windings. Hence, this technique contradicts the need for high accuracy linear caliper of compact size. Alternatively, the transmitter field can be confined with magnetically permeable materials so that the effectiveness of a given degree of separation is increased. However, this technique leads to additional complexity, cost, and sensitivity to external fields, in a practical device.
Furthermore, the simple winding configurations disclosed in association with these techniques include no means for creating a device with a measuring range significantly exceeding the span of the transmitter and receive windings. In addition, the simple winding configurations provide no means for significantly enhancing the degree of output signal change per unit of displacement for a given measuring range. Thus, the practical measuring resolution of these devices is limited for a given measuring range.
The need for a high accuracy inductive linear caliper which rejects both extraneous signal components and external fields, is compact, of simple construction, and capable of high resolution measurement over an extended measuring range without requiring increased fabrication and circuit accuracies, has therefore not been met previously.